Monostable ferroelectric active matrix display

ABSTRACT

The monostable ferroelectric active matrix display comprises a liquid-crystal layer in the form of a monodomain having an unambiguously defined direction of the layer normal z of the smC* phase, the liquid-crystal layer preferably having a chevron C-1, chevron C-2 or bookshelf geometry.

Replacement of the cathode ray tube with a flat panel screen requires adisplay technology which simultaneously makes it possible to achieve ahigh image resolution, i.e. more than 1000 lines, a high imagebrightness (>200 cd/m²), a high contrast (>100:1), a high frame rate(>60 Hz), an adequate color representation (>16 million colors), a largeimage format (screen diagonal >40 cm), a low power consumption and awide viewing angle, at low production costs. At present, there is notechnology which fully satisfies all these features simultaneously.

Many manufacturers have developed screens which are based on nematicliquid crystals and have been used in recent years in the field ofnotebook PCs, Personal Digital Assistants, desktop monitors etc. Use ismade here of the technologies STN (supertwisted nematics), AM-TN (activematrix—twisted nematics) AM-IPS (active matrix—in-plane switching) andAM-MVA (active matrix—multidomain vertically aligned), which aredescribed in the relevant literature; see, for example, T. Tsukuda,TFT/LCD: Liquid Crystal Displays Addressed by Thin-Film Transistors,Gordon and Breach, 1996, ISBN 2-919875-01-9, and the references citedtherein; SID Symposium 1997, ISSN-0097-966X pages 7 to 10, 15 to 18, 47to 51, 213 to 216, 383 to 386, 397 to 404 and the references citedtherein. Furthermore, use is being made of the technologies PDP (plasmadisplay panel), PALC (plasma addressed liquid crystal), ELD(electroluminescent display), FED (field emission display) etc., whichare also explained in the above-cited SID report.

Clark and Lagerwall (U.S. Pat. No. 4,367,924) have been able to showthat the use of ferroelectric liquid crystals (FLCs) in very thin cellsresults in opto-electrical switching or display elements which haveresponse times which are faster by a factor of up to 1000 compared withconventional TN (“twisted nematic”) cells (see, for example, EP-A 0 032362). Owing to this and other favorable properties, for example thepossibility of bistable switching and the fact that the contrast isvirtually independent of the viewing angle, FLCs are basically suitablefor areas of application such as computer displays and TV sets, as shownby a monitor marketed in Japan by Canon since May 1995.

The use of FLCs in electro-optical or fully optical components requireseither compounds which form smectic phases and are themselves opticallyactive, or the induction of ferroelectric smectic phases by dopingcompounds which, although forming such smectic phases, are notthemselves optically active, with optically active compounds. Thedesired phase should be stable over the broadest possible temperaturerange.

The individual pixels of an LC display are usually arranged in an x,ymatrix formed by the arrangement of a series of electrodes (conductortracks) along the rows and a series of electrodes along the columns onthe upper or lower side of the display. The points of intersection ofthe horizontal (row) electrodes and the vertical (column) electrodesform addressable pixels.

This arrangement of the pixels is usually referred to as a passivematrix. For addressing, various multiplex schemes have been developed,as described, for example, in Displays 1993, Vol. 14, No. 2, pp. 86-93,and Kontakte 1993 (2), pp. 3-14. Passive matrix addressing has theadvantage of simpler display production and consequently lowerproduction costs, but the disadvantage that passive addressing can onlybe carried out line by line, which results in the addressing time forthe entire screen with N lines being N times the line addressing time.For usual line addressing times of about 50 microseconds, this means ascreen addressing time of about 60 milliseconds in, for example, theHDTV (high definition TV, 1152 lines) standard, i.e. a maximum framerate of about 16 Hz, too slow for moving images. In addition, display ofgray shades is difficult. At the FLC Conference in Brest, France (Jul.20-24, 1997, see Abstract Book 6^(th) International Conference onFerroelectric Liquid Crystals, Brest/France), a passive FLC display withdigital gray shades was shown by Mizutani et al., in which each of theRGB pixels (RGB=red, green, blue) was divided into sub-pixels, allowingthe display of gray shades in digital form through partial switching.Using three basic colors (red, green, blue), N gray shades result in3^(N) colors. The disadvantage of this method is the considerableincrease in the number of screen drivers necessary and thus in thecosts. In the case of the display shown in Brest, three times the numberof drivers were necessary than in a standard FLC display without digitalgray shades.

In so-called active matrix technology (AMLCD), a nonstructured substrateis usually combined with an active matrix substrate. An electricallynon-linear element, for example a thin-film transistor, is integratedinto each pixel of the active matrix substrate. The nonlinear elementscan also be diodes, metal-insulator-metal and similar elements, whichare advantageously produced by thin-film processes and are described inthe relevant literature; see, for example, T. Tsukuda, TFT/LCD: LiquidCrystal Displays Addressed by Thin-Film Transistors, Gordon and Breach,1996, ISBN 2-919875-01-9, and the references cited therein.

Active matrix LCDs are usually operated with nematic liquid crystals inTN (twisted nematics), ECB (electrically controlled birefringence), VA(vertically aligned) or IPS (in-plane switching) mode. In each case, theactive matrix generates an electric field of individual strength on eachpixel, producing a change in alignment and thus a change inbirefringence, which is in turn visible in polarized light. A severedisadvantage of these processes is the poor video capability owing toexcessively slow response times of nematic liquid crystals.

For this and other reasons, liquid crystal displays based on acombination of ferroelectric liquid crystal materials and active matrixelements have been proposed, see for example WO 97/12355, orFerroelectrics 1996, 179, 141-152, W. J. A. M. Hartmann, IEEE Trans.Electron. Devices 1989, 36 (9; Pt. 1), 1895-9.

Hartmann utilized a combination of the so-called “quasi-bookshelfgeometry” (QBG) of an FLC and a TFT (thin-film transistor) active matrixto simultaneously achieve high response speed, gray shades and hightransmission. However, the QBG is not stable over a broad temperaturerange, since the temperature dependence of the smectic layer thicknessdisrupts or rotates the field-induced layer structure. Moreover,Hartmann utilizes an FLC material having a spontaneous polarization ofmore than 20 nC/cm², which, for pixels having realistic dimensions of,for example, an area of 0.01 mm², leads to high electric charges (atsaturation, Q=2 A P, A=pixel area, P=spontaneous polarization). Withlow-cost amorphous silicium TFTs, for example, these high charges cannotreach the pixel in the course of the opening time of the TFT. For thesereasons, this technology has not been pursued any further to date.

While Hartmann utilizes the charge-controlled bistability to display avirtually continuous gray scale, Nito et al. have suggested a monostableFLC geometry (see Journal of the SID, 1/2, 1993, pages 163-169) in whichthe FLC material is aligned by means of relatively high voltages suchthat only a single stable position results from which a number ofintermediate states are generated by application of an electric fieldvia a thin-film transistor. These intermediate states correspond to anumber of different brightness values (gray shades) when the cellgeometry is matched between crossed polarizers.

One disadvantage of this technique is the occurrence of a streakytexture in the display which limits contrast and brightness of this cell(see FIG. 8 in the abovementioned citation). While it is possible tocorrect the disadvantageous streaky texture by treatment with a highelectric voltage (20-50 V) in the nematic or cholesteric phase (see page168 of the abovementioned citation), such a field treatment isunsuitable for mass production of screens and usually does not result intemperature-stable textures. Furthermore, this method produces switchingonly in an angle range of up to a maximum of once the tilt angle, whichis about 22° in the case of the material used by Nito et al. (cf. p.165, FIG. 6) and thus produces a maximum transmission of only 50% of thetransmission of two parallel polarizers.

The object of the present invention is to provide a ferroelectric activematrix liquid crystal display comprising a ferroelectric liquid crystalmixture, where the liquid crystal mixture assumes a monostable position,but without forming a streaky texture, is temperature-stable and makesit possible to achieve a very high maximum transmission and a very highcontrast.

This object is achieved according to the invention by a monostableferroelectric active matrix display comprising a liquid-crystal layer,preferably in chevron C2 geometry, in the form of a monodomain having anunambiguously defined direction of the layer normal z of the chiralsmectic phase, the ratio between the sum of pretilt angle and layerleaning angle and tilt angle (AR=(LLA+PTA)/TIA) being greater than 0.1and the absolute value of the dielectric anisotropy DA being preferablyless than 3.

The spontaneous polarization P in the liquid-crystal layer is preferablybetween 0.1 and 15 nC/cm².

The tilt angle TIA in the liquid-crystal layer is preferably between 9and 40°.

The ratio between the product of anchoring strength and the sinus of thetilt angle and the spontaneous polarization (AS sin TIA/P) is preferablyless than 20 V/μm.

The object is furthermore achieved by a monostable ferroelectric activematrix display comprising a liquid-crystal layer in chevron C1 geometryin the form of a monodomain having an unambiguously defined direction ofthe layer normal z of the smC* phase, the pretilt angle PTA being atleast 5° and the ratio between pretilt angle and layer leaning angle(PTA/LLA) being greater than 0.7.

The object is furthermore achieved by a monostable ferroelectric activematrix display comprising a liquid-crystal layer in bookshelf geometryin the form of a monodomain having an unambiguously defined direction ofthe layer normal z of the smC* phase, the pretilt angle PTA being atleast 1°.

The object is furthermore achieved by a monostable ferroelectric activematrix display comprising a liquid-crystal layer in the form of amonodomain having an unambiguously defined direction of the layer normalz of the smC* phase having the following properties:

a spontaneous polarization P of between 0.1 and 15 nC/cm²,

a tilt of 9° to 45° C.

an absolute value of the ratio between layer leaning angle and tiltangle of at least 0.2,

a pitch in the chiral nematic (cholesteric) phase of at least 50 μmwithin the temperature range of 5° C. above the smectic-nematic phasetransition or, if the range of existence of the nematic (cholesteric)phase is less than 5° C., within a temperature range of at least 80% ofthe nematic phase range, and

an absolute value of the dielectric anisotropy DA of less than 3.

The liquid-crystal layer preferably has one or more, in particular all,of the following features:

the angle between the layer normal z of the smC* phase and thepreferential direction n of the nematic or cholesteric phase (N* phase)is in the range from 0.5 to 1.0 times the smC* tilt angle, but at least5°,

the ferroelectric liquid-crystal layer has the phase sequence

I*−N*−smC*

where an smA* phase having a range of existence of not more than 20°,preferably not more than 1°, may exist between the N* phase and the smC*phase.

The active-matrix FLCD of the invention preferably comprises, asoptically active layer, a ferroelectric liquid-crystalline medium(liquid-crystal phase) having a phase sequence of

isotropic−nematic or cholesteric (N*)−smectic C*

or a phase sequence of

isotropic−nematic or cholesteric (N*)−smectic A*−smectic C*,

where the smectic A* phase has a range of existence (please range) ofnot more than 2° C., preferably not more than 1° C., particularlypreferably not more than 0.5° C. The asterisk (*) attached to the phasename indicates a chiral phase.

The displays are preferably produced by a process, which comprisesintroducing the liquid-crystal layer into the space between a rubbedupper substrate plate and a rubbed lower substrate plate of the activematrix display, the rubbing directions on the upper substrate plate andthe lower substrate plate being essentially parallel, and cooling theliquid crystal phase from the isotropic phase, an electric voltage beingapplied to the display at least during the N*→smC* or N*→smA*→smC* phasetransition.

The FLC mixture is filled into an active matrix display. Production andcomponents of an AM display of this type are described in theabove-cited Tsukuda reference. However, in contrast to nematic displays,the thickness of the FLC layer is only from 0.7 to 2,5 μm, preferably1-2 μm. Moreover, the rubbing directions on upper and lower substrateplates are essentially parallel. The term “essentially parallel”includes antiparallel rubbing directions or rubbing directions which areweakly crossed, i.e. up to 10%.

It is important for the operation of this display that in the productionof the display, during controlled cooling, a direct electric current,preferably of less than 5V, is applied and maintained during the N*→smC*or N*→smA*→smC* phase transition, with the result that the whole displayassumes a monostable monodomain which appears completely dark betweencrossed polarizers.

After this domain has been obtained, the direct current is switched off.In contrast to the abovementioned approach by Hartmann or conventionalbistable FLCDs, the resulting texture is monostable. This means that thepreferred n director (which indicates the preferential direction of thelong axes of the molecules) is in the rubbing direction of the cell,whereas the z director (which indicates the preferential direction ofthe smectic layer normal) is oblique relative to the rubbing directionby approximately the tilt angle value. This constellation is exactly theopposite of the conventional bistable cell according to Clark andLagerwall in which the z director is in the rubbing direction.

In contrast to Nito's approach, this is exactly the orientation in whichthere are no two layer normals and no two orientation domains, whichultimately lead to the unwanted streaky texture described above, but asingle unambiguous direction of the z director and thus a singlemonodomain only. Furthermore, it is possible to obtain twice the tiltangle, which leads to 100% transmission, based on parallel polarizers,i.e. double brightness is achieved.

At a suitable angle of rotation, the resulting display appearscompletely dark between crossed polarizers. On applying an addressingvoltage of only a few volts, the display appears bright, it beingpossible to vary the brightness continuously by means of the voltage,and is almost as bright as two parallel polarizing films when saturated.The angle between the preferential direction of the nematic (orcholesteric) phase and the layer normal (z director) is ideally and thuspreferably equal to the tilt angle of the smectic C phase, or at leastessentially equal to the tilt angle. For the purposes of the invention,“essentially” means preferably a range from half the tilt angle to thefull tilt angle, particularly preferably from 0.5 to 1.0 times the tiltangle, but at least 5°.

The ferroelectric active matrix liquid crystal display of the inventionis particularly useful in practice, in particular for TV, HDTV ormultimedia, since it combines high transmission, short response times,gray scale and thus full color capability, low-cost production and abroad temperature range. Furthermore, the display can be operated atvoltages of ≦10 volts, preferably of ≦8 V, particularly preferably of ≦5V.

For display of gray shades or as many natural colors as possible, thecharacteristic line (transmission plotted against voltage) of the liquidcrystal mixture should be sufficiently flat to address the gray shadesreliably using the available voltages, and the saturation voltage shouldnot be too high.

The saturation voltage V90, at which 90% of maximum transmission areachieved, should not be too high so as to allow operation of the displayat below 30 V, preferably below 15 V, more preferably below 10 V,particularly preferably below 8 V, especially below 5 V. The thresholdvoltage V10 should preferably be adapted to V90 so as to ensure that thethe characteristic line width CLW is large enough to be able to addressa sufficiently high number of gray shades. This is generally the case ifthe characteristic line width CLW=V90−V10 is at least 100 mV, preferablyat least 200 mV, more preferably at least 500 mV, particularlypreferably at least 1 V, especially at least 1.5 V.

Furthermore, the maximum transmission of the cell should be at least 40%(based on an empty cell between two parallel polarizing films),preferably at least 50%, more preferably at least 70%, particularlypreferably at least 80%, especially at least 90%. Moreover, the T,Vcharacteristic line should preferably increase strictly monotonically(with increasing voltage). A fall in transmission after reaching atransmission maximum is undesirable.

The invention accordingly provides the selection of liquid crystals andmixtures thereof having suitable material parameters for advantageousadjustment of the characteristic line.

The invention furthermore provides a monostable active matrix FLCdisplay, in which an optimum characteristic line is achieved byselection of liquid crystals or mixtures thereof by specific combinationof a plurality of material properties of the ferroelectric liquidcrystal, and the use of liquid crystal mixtures having these propertiesfor active matrix FLC displays.

In particular, the term “active matrix display” as used herein includesan LCD in which one of the two substrates is replaced by the rear sideof an IC chip (IC=integrated circuit) as described, for example, in D.M. Walba, Science 270, 250-251 (1995).

The abbreviations used in the examples and in the description of theinvention are explained in the table below.

Definitions and abbreviations (abbr.): Term Abbr. Unit Anchoringstrength AS J/m³ Tilt angle TIA Degrees Pretilt angle PTA Degrees Layerleaning angle LLA Degrees Layer rotation angle LRA Degrees Spontaneouspolarization P nC/cm² Threshold voltage V10 Volt Saturation voltage V90Volt Dielectric anisotropy DA — Optical anisotropy OA — Characteristicline width (=V90-V10) CLW Volt Characteristic line (optical transmissionas a — — function of voltage) Chevron 1 geometry (LLA > 0) C1 — Chevron2 geometry (LLA < 0) C2 — Bookshelf geometry (ideally, LLA = 0) BS —Pitch pitch μm Isotropic phase 1 — Nematic or chiral nematic orcholesteric N* — phase (these terms are used synonymously here) SmecticA phase or chiral smectic A phase smA* — Chiral smectic C* phase smC* —Cell thickness d μm Electric field (=voltage/cell thickness) E V/μm Freeenergy density g J/m³ Saturation transmission Tsat % Opticaltransmission T % Angle relation (AR = (LLA + PTA)/TIA) AR %

The C1, C2 geometries etc. are described by D. C. Ulrich and S. J.Elston in Ferroelectrics, vol. 178, p. 177-186 (1996).

The characteristic line of the ferroelectric liquid crystal cell of theinvention is influenced by a number of parameters which, alone or incombination, should be in preferred ranges so that optimum switichingconditions are achieved. These parameters are in particular thespontaneous polarization (P), the tilt angle (TIA), the layer leaningangle (LLA), the pretilt angle (PTA), the anchoring strength (AS), thedielectric anisotropy (DA), furthermore the layer rotation angle (LRA),the cell thickness (d), the pitch of the cholesteric phase and thesmectic C* phase and the optical anisotropy (OA).

It is found that all these parameters influence the characteristic line,although to a varying extent. The characteristic line (T,V) shouldpreferably have the following characteristics.

In contrast to all usual experiences made with ferroelectric LCDs, ithas been found that the characteristic line is not influenced, orinfluenced only to a small extent, by the rotational viscosity; instead,e.g. the saturation voltage V90 is strongly dependent on the spontaneouspolarization (P) and the anchoring strength (AS).

The spontaneous polarization (P) should preferably be between 0.1 and 15nC/cm² (here, this always means the absolute value of P), preferablybetween 0.2 and 10 nC/cm², more preferably between 0.4 and 8 nC/cm²,particularly preferably between 0.5 and 6 nC/cm², especially between 0.8and 3.5 nC/cm².

The tilt angle should preferably be in the range from 9° to 45°,preferably between 12° and 35°, more preferably between 14° and 31°,particularly preferably between 17° and 27°, especially between 19° and25°.

The layer rotation angle (LRA=angle between the preferential directionof the nematic phase and the smC* layer normal) should preferably be atleast 5°.

The absolute value of the dielectric anisotropy should preferably beless than 3 (three), more preferably less than 2.5, particularlypreferably less than 1.8, especially less than 1.2.

The product of anchoring strength (AS) and the sinus of the tilt angledivided by the spontaneous polarization (P) should preferably be lessthan 20 V/μm, preferably less than 15 V/μm, more preferably less than 12V/μm, particularly preferably less than 9 V/μm, especially less than 6V/μm.

The parameter indications relate to at least one temperature in theoperating range of the ferroelectric liquid crystal display.

The display of the invention can be operated not only in the range ofthe smectic C* phase, but also—at least partially—in the range ofanother tilted smectic phase, with the abovementioned properties beingapplied by analogy.

The monostable active matrix FLC display of the invention can beoperated in chevron C1 geometry, chevron C2 geometry or bookshelf orquasi-bookshelf geometry, respectively. For all three geometries, thepreferred combination of ranges of spontaneous polarization and tiltangle values is valid:

Spontaneous polarization Tilt angle TIA P Range in nC/cm² Range indegrees Preferably <15  9-45 More preferably 0.2-10 12-35 Particularlypreferably 0.4-8 14-31 Very particularly preferably 0.5-6 17-27Especially 0.8-3.5 19-25

Even more preferred are abovementioned combinations of P and TIAtogether with an absolute value of the dielectric anisotropy which isless than 3 (three), more preferably less than 2 (two), particularlypreferably less than 1.5, especially less than 1.2.

The pitch of the cholesteric helix should be at least 50 μm within thetemperature range of 5° above the smectic phase transition or, if therange of existence of the cholesteric phase is smaller, preferablywithin a temperature range of 80% of this range of existence. It ispreferred to achieve a pitch of at least 70 μm, particularly preferablyof at least 100 μm, to achieve high contrasts.

In the C2 geometry, which is usually preferred at small pretilt anglesand completely disappears at large pretilt angles in the limiting case(PTA>TIA), the layer leaning angle (LLA) together with the pretilt angle(PTA) should preferably relate to the tilt angle (TIA) as follows: theratio AR between the sum of pretilt angle and layer leaning angle andthe tilt angle (i.e. (LLA+PTA)/TIA) should generally be at least 0.1,preferably at least 0.15, more preferably at least 0.25, particularlypreferably at least 0.5, especially at least 0.7. Here, the absolutevalue of the dielectric anisotropy DA should be less than 3.

Most particularly, advantageous characteristic lines are achieved usingthe combinations listed in the table below:

Tilt Spontaneous angle TIA polarization P Range in C2 geometry Range innC/cm² degrees AR range Preferably <15  9-45 greater than 0.1 Morepreferably 0.2-10 12-35 greater than 0.15 Particularly preferably 0.4-814-31 greater than 0.3 Very particularly 0.5-6 17-27 greater than 0.5preferably Especially 0.8-3.5 19-25 greater than 0.7

Even more preferred are abovementioned combinations of P and TIAtogether with an absolute value of the dielectric anisotropy which isless than 3, more preferably less than 2, particularly preferably lessthan 1.5, especially less than 1.2.

The C2 geometry is generally preferred over C1 and bookshelf.

In the C1 geometry, which is preferred at large pretilt angles, thepretilt angle should be at least 5°, at a suitable layer leaning angle,since otherwise no switching occurs at low voltage.

At a suitable layer leaning angle, the relation between layer leaningangle (LLA) and pretilt angle (PTA) should advantageously be as follows:the ratio between pretilt angle and layer leaning angle should begreater than 0.7.

Most particularly, advantageous characteristic lines are achieved usingthe combinations listed in the table below (for C1 geometry):

Tilt Spontaneous angle TIA polarization P Range in Range in nC/cm²degrees PTA/LLA range Preferably <15  9-45 greater than 0.25 Morepreferably 0.2-14 12-35 greater than 0.3 Particularly preferably 0.3-1214-31 greater than 0.33 Very particularly 0.4-9 17-27 greater than 0.4preferably Especially 0.5-7 19-25 greater than 0.5

Even more preferred are abovementioned combinations of P and TIAtogether with a dielectric anisotropy which is greater than −1,preferably with a positive dielectric anisotropy, i.e. >0.

In the bookshelf geometry, which is herein defined for the range−5°<LLA>+5° (ideally LLA=0), the pretilt angle should generally be atleast 1°, preferably at least 2°. Particulary preferably, the pretiltangle should be at least 1°, the spontaneous polarization should be atleast 0.1 nC/cm² and not more than 15 nC/cm², and the tilt angle shouldbe at least 12°.

In the tables above, the spontaneous polarization can also preferably befrom 0.1 to 15 nC/cm².

A mixture which is particularly suitable for use in the display of theinvention comprises at least six (6), preferably at least eight (8),particularly preferably at least nine (9), especially at least eleven(11), components which are selected such that the spontaneouspolarization is between 0.1 and 15 nC/cm², the tilt angle is between 17and 27°, the ratio between layer leaning angle and tilt angle is atleast 0.3, the pitch of the chiral nematic phase is at least 50 μm(range of 5° C. above the phase transition), the absolute value of thedielectric anisotropy is less than 3, and the phase sequence is

 isotropic−nematic−smectic C*

or

isotropic−nematic−smectic A−smectic C*,

where the smectic A phase has a range of existence of not more than 2°C.

The examples which follow illustrate the invention.

EXAMPLES

1. Preparation of an aligned, monostable FLC cell:

A glass substrate coated with transparent conductive indium-tin oxide isstructured in a photolithographic process to give an electrode pattern.The transparent conductor tracks of this electrode pattern are used forelectrical addressing of the display by menas of a function generator,thus simulating the switiching behavior of a thin-film transistor. Twoglass plates structured in this way, forming the top and bottom of thedisplay—i.e. the outer plates—are provided with alignment layers whichare rubbed and joined with the aid of an adhesive frame with addition ofa concentration of 0.5% by weight of spacer beads having a diameter of1.3 μm. The adhesive is hardened by careful heating, the liquid-crystalmixture is filled in at 100° C. by capillary forces, and the cell isslowly cooled to a temperature above the I-N* phase transition. At thistemperature, a direct voltage of 4 V is applied and the cooling processis continued until a temperature of 22° C. is reached. The directvoltage is then switched off. A monostable monodomain is obtained whichappears completely dark between crossed polarizers.

2. Determination of the characteristic line (T,V characteristic):

Voltages of variable amplitude are applied to the cell by means offunction generator (Wavetech model) and amplifier (from Krohn-Hite)using monopolar pulses of 10 ms duration at 30 ms intervals, and thetransmission is measured by means of a photodiode and an oscilloscope.In this way, the transmission is obtained (as a photodiode signal) as afunction of voltage. This characteristic line usually exhibits asaturation of transmission at high voltages; this value is denoted 100%.V10 is the threshold voltage at which the brightness is exactly 10% ofthe saturation transmission, V90 is the voltage at which 90% of thesaturation transmission are achieved.

3. Determination of anchoring strength (AS)

The empirical anchoring strength AS is calculated from the V90 valueobtained in Example 2, the layer thickness and the spontaneouspolarization according to the formula below:

AS(in J/m³)=10 *V90 (in V)*P(in nC/cm²)/sin(TIA)*d(in μm)

Once the AS value has been determined for one mixture and one cell type,it is possible to vary P, d or other parameters, provided the chemicalnature of the composition of the mixture is not changed too much andthus the AS value can be applied to the design of an AM-FLC display.

4. An FLC display having a cell thickness of 1.25 μm is filled with anFLC mixture which has the phase sequence I-N*−smC* and the followingphysical data:

Geometry: C2 Tilt angle: 21° Layer leaning angle: 18° LLA/TIA: 0.86Pretilt angle:  0° Optical anisotropy: 0.17,

and aligned.

Variation of P and AS yields the following T,V characteristics(characteristic lines):

P AS V10 V90 CLW = V90-V10 No. nC/cm² J/m³ V V V A 0.5 83 2.5 8.0 5.5 B2 195 2.4 4.4 2.0 C 4 195 0.8 2.2 1.4 D 8 195 0.5 1.1 0.5 E 16 195 0.20.5 0.3 F 0.1 7 1.2 3.6 2.4

Advantageous switching is possible in a P range from 0.1 to 15 nC/cm²,at low P values, however, preferably combined with weak anchoring.

5. An FLC display having a cell thickness of 1.25 μm is filled with anFLC mixture which has the phase sequence I-N*−smC* and the followingphysical data:

Geometry: C2 Pretilt angle: 0° Optical anisotropy: 0.17,

and aligned.

Variation of the tilt angle TIA yields the following characteristiclines:

P LLA TIA V10 V90 CLW = V90-V10 Tsat No. nC/cm² Deg. Deg. V V V % A 0.518 21 2.5 8.0 5.5 68 B 3 18 24 0.6 1.5 0.9 72 C 3 18 27 0.7 1.7 1.0 70 D3 18 30 0.8 1.8 1.0 52 E 1.5 5 35 3.0 3.3 0.3  34* F 1.5 30 35 0.8 4.53.7  55** G 1.5 10 12 0.3 1.2 0.9 42 H 1.5 5 10 0.7 1.5 0.8 60 I 1.5 1015 0.6 1.8 1.2 58 J 1.5 14 15 0.5 1.9 1.4 56 K 1.5 3 9 0.3 0.8 0.5 28 L1.5 35 40 0.5 5.0 4.5 50 *here, pretilt = 5°, **with intermediatemaximum at 5 V/73%.

Advantageous switching is possible in a TIA range from 10° to 35°. Inthese examples, the product of optical anisotropy and cell thickness wasnot optimized, resulting in a maximum transmission of only about 80%.

By adapting e.g. the cell thickness, all maximum transmissions obtainedhere can be increased by a factor of 1.25, giving almost 100% with thepreferred configurations.

6. An FLC display having a cell thickness of 1.25 μm is filled with anFLC mixture which has the phase sequence I-N*−smA* (range of existence:<2°)−smC* and the following physical data:

Geometry: C2 Dielectric anisotropy: −1 Optical anisotropy: 0.17,

and aligned.

Variation of LLA and PTA yields the following T,V characteristics(characteristic lines); AR=(LLA+PTA)/TIA:

P LLA TIA PTA AR V10 V90 CLW = V90-V10 No. nC/cm² Deg. Deg. Deg. — V V VA 1.5 5 25 20 1.00 1.2 3.0 1.8 B 1.5 5 25 5 0.40 1.7 2.8 1.1 C 1.5 5 255 0.40 3.3 5.0 1.7 D 1.5 20 25 5 1.00 2.2 6.0 3.8 E 1.5 5 35 5 0.28 2.93.4 0.5 F 2.0 0 21 3 0.13 1.2 1.5 0.3 G 2.0 1 30 1 0.07 — — no switchingH 2.0 1 31 2 0.096 — — no switching J 2.0 5 21 0 0.24 2.4 3.1 0.7

Advantageous switching is possible in a range of AR>0.1. At lowervalues, the characteristic line is too steep to realize gray shades, oreven no switching at all is observed.

7. An FLC display having a cell thickness of 1.25 μm is filled with anFLC mixture which has the phase sequence I-N*−smC* and the followingphysical data:

Geometry: C2 Tilt angle: 25° Layer leaning angle: 15° LLA/TIA: 0.60Pretilt angle: 0° Dielectric anisotropy: −1 Optical anisotropy: 0.17Spontaneous polarization: 1,5 nC/cm²,

and aligned.

Variation of AS yields the following T,V characteristics (characteristiclines):

AS V10 V90 CLW = V90-V10 No. J/m³ V V V A 74 1.2 2.6 1.4 B 159 2.5 5.63.1 C 43 0.6 1.5 0.9 D E 2 0.2 0.7 0.5* F *here, Ps = 0.75 nC/cm²

Advantageous characteristic lines are obtained at low values ofAS*sin(TIA)/P, in particular at AS*sin(TIA)/P<20 V/μm, preferably <16μm.

8. An FLC display having a cell thickness of 1.25 μm is filled with anFLC mixture which has the phase sequence I-N*−smC* and the followingphysical data:

Geometry: C2 Tilt angle: 22° Layer leaning angle: 15° LLA/TIA: 0.682Pretilt angle: 0° Optical anisotropy: 0.17 Spontaneous polarization: 0,5nC/cm²,

and aligned.

V10 V90 Tsat No. DA V V % Notes A +3.2 — — 4% virtually no switching B−3.1 — — 9% virtually no switching C 0 3 8 78% D +0.5 0.8 2.0 75% here,Ps = 2 nC/cm²

Adavantageous characteristic lines are obtained at low absolute valuesof the dielectric anisotropy, in particular at |DA|<3, preferably at|DA|<2, particularly preferably at |DA|<1.5, especially at |DA|<1.2.

9. An FLC display having a cell thickness of 1.25 μm is filled with anFLC mixture which has the phase sequence I-N*−smC* and the followingphysical data:

Geometry: C1 Spontaneous polarization: 1.5 nC/cm² Tilt angle: 20° Layerleaning angle: 15° (C1, leaning direction different to C2) LLA/TIA: 0.75Dielectric anisotropy: −1 Optical anisotropy: 0.17,

and aligned.

Variation of the pretilt angle yields the following T,V characteristics(characteristic lines):

PTA V10 V90 CLW = V90-V10 No. Degrees V V V A 0 no switching B 10 noswitching C 20 1.6 1.8 0.2 D 25 2.2 2.5 0.3 E 30 2.3 3.1 0.8 F 40 2.44.0 1.6

In C1 geometry, no switching is observed at small pretilt angles. At anacceptable minimum value of the tilt angle of more than 9°, a pretilt ofat least 5° is required to obtain a useful characteristic line.

If the layer leaning angle is adjusted to less than 5° (absolute value),switching is only observed at a pretilt angle of at least one degree.

PTA is at least 5°; PTA/LLA is greater than 0.7.

What is claimed is:
 1. A monostable ferroelectric active matrix displaycomprising a liquid-crystal layer in the form of a monodomain having anunambiguously defined direction of the layer normal z of the smC* phase,the ratio between the sum of pretilt angle and layer leaning angle andthe tilt angle (AR=(LLA+PTA)/TIA) being greater than 0.1 and theabsolute value of the dielectric anisotropy DA being less than
 3. 2. Anactive matrix display as claimed in claim 1 wherein the spontaneouspolarization P in the liquid-crystal layer is less than 15 nC/cm2.
 3. Anactive matrix display as claimed in claim 1 wherein the tilt angle TIAin the liquid-crystal layer is between 9 and 40°.
 4. An active matrixdisplay as claimed in claim 1 wherein the ratio between the product ofanchoring strength and the sinus of the tilt angle and the spontaneouspolarization (AS sin TIA/P) is less than 20 Vμm.
 5. An active matrixdisplay as claimed in claim 1 wherein the liquid-crystal layer has oneor more of the following properties: the angle between the layer normalz of the smC* phase and the preferential direction n of the nematic orcholesteric phase (N* phase) is in the range from 0.5 to 1.0 times thesmC* tilt angle, but at least 5°, the ferroelectric liquid-crystal layerhas the phase sequence I*−N*−smC* where an smA* phase having a range ofexistence of not more than 2° may exist between the N* phase and thesmC* phase.
 6. A process for producing an active matrix display asclaimed in claim 1 which comprises introducing the liquid-crystal layerinto the space between a rubbed upper substrate plate and a rubbed lowersubstrate plate of the active matrix display, the rubbing directions onthe upper substrate plate and the lower substrate plate beingessentially parallel, and cooling the liquid crystal phase from theisotropic phase, an electric voltage being applied to the display atleast during the N*smC* or N*smA*smC* phase transition.
 7. An activematrix display obtainable by the process as claimed in claim
 6. 8. Amonostable ferroelectric active matrix display comprising aliquid-crystal layer in chevron C1 geometry in the form of a monodomainhaving an unambiguously defined direction of the layer normal z of thesmC* phase, the pretilt angle PTA being at least 5° and the ratiobetween pretilt angle and layer leaning angle (PTA/LLA) being greaterthan 0.7.
 9. A monostable ferroelectric active matrix display comprisinga liquid-crystal layer in bookshelf geometry in the form of a monodomainhaving an unambiguously defined direction of the layer normal z of thesmC* phase, the pretilt angle PTA being at least 1°.
 10. A monostableferroelectric active matrix display comprising a liquid-crystal layer inthe form of a monodomain having an unambiguously defined direction ofthe layer normal z of the smC* phase having the following properties: aspontaneous polarization P of between 0.1 and 15 nC/cm2, a tilt angleTIA of 9° to 45° C. an absolute value of the ratio between layer leaningangle and tilt angle of at least 0.2, a pitch in the chiral nematic(cholesteric) phase of at least 50 μm within the temperature range of 5°C. above the smectic-nematic phase transition, and an absolute value ofthe dielectric anisotropy DA of less than 3.